Method and device for monitoring the access to the cardiovascular system of a patient

ABSTRACT

A device for monitoring the access to the cardiovascular system of a patient undergoing an extracorporeal treatment of blood in a machine ( 1 ) comprising a treatment device ( 4 ) and an extracorporeal circuit ( 2 ), comprises: a voltage generator ( 16 ) for generating a potential difference between a part of the machine ( 1 ) and a first point (B) of a venous branch ( 8 ) of the extracorporeal circuit ( 2 ), connecting the patient to the treatment device ( 4 ); a detector ( 17 ) for detecting the value (dV) of a quantity that correlates with the electric current along at least one section ( 10   a;    10   b;    10   c ) of the venous branch ( 10 ) between the first point (B) and a venous needle ( 13 ) fitted at the end of the venous branch ( 8 ) and inserted in the vascular system of the patient (P); calculating means ( 15 ) for comparing the detected value (dV) with a reference range (T).

[0001] The present invention relates to a method and a device formonitoring the access to the cardiovascular system of a patientundergoing an extracorporeal treatment of blood.

[0002] The invention is useful in any kind of treatment in which bloodis continuously withdrawn from a patient, circulated and treated in atreatment device, and returned, once treated, to the patient.Hemodialysis, hemofiltration, apheresis and plasmapheresis are examplesof such treatment.

[0003] For the sake of clarity, the invention will be describedhereunder in relation to a specific treatment, hemodialysis, to whichhowever it is not limited as will readily appear to the persons skilledin the art.

[0004] A dialysis machine generally comprises.

[0005] a filter (hemodialyzer) having a first and a second compartmentsseparated from one another by a semipermeable membrane;

[0006] an extracorporeal blood circuit, having an arterial branchconnected to an inlet of the first compartment and a venous branchconnected to an outlet of the first compartment; a blood pump isarranged on the arterial line and a bubble trap is connected to thevenous line;

[0007] an dialysis liquid circuit, having an fresh dialysis liquidsupply branch connected to an inlet of the second compartment and a usedliquid branch connected to an outlet of the second compartment.

[0008] In use, the blood of the patient and the dialysis liquid arerespectively circulated in the first and the second compartments,generally in counterflow.

[0009] During a dialysis treatment, undesirable substances (by-productsof the metabolism, such as urea, creatinine, etc.) contained in theblood migrate across the semipermeable membrane from the bloodcompartment to the dialysis liquid compartment by diffusion (dialysisphenomenon, strictly speaking) and also generally by convection, afraction of plasma water being usually filtered during the treatment sothat the patient looses a few kilograms (so-called “weight loss”)corresponding to an excess of water accumulated in the body between twotreatment sessions.

[0010] Each branch of the extracorporeal circuit is fitted with a needle(respectively, arterial needle and venous needle), by means of which theextracorporeal circuit is connected to the patient: just before startingthe treatment, the arterial needle and the venous needle are inserted inthe fistula of the patient (portion of a vein surgically connected to anartery) for respectively collecting the blood to be treated andreturning the treated blood to the patient's cardiovascular system.

[0011] Disconnection of one of the aforementioned needles from thefistula causes interruption of access to the patient's cardiovascularsystem. Disconnection of the venous needle, if not detected in time, hasparticularly serious consequences, as it can cause exsanguination of thepatient. For this reason there have been various attempts to providemethods capable of detecting disconnection of the needles, andespecially of the venous needle.

[0012] One of these methods, which is based on the electricalconductivity of the blood, is described in WO 99/12588. According tothis method, the extracorporeal circuit and the patient's cardiovascularsystem are subjected to an electric current, and changes in the currentthat are caused by the disconnection of one of the needles or both ofthe needles is detected, by means of measuring instruments arrangedalong the extracorporeal circuit. The measuring instrument used areinductive couplers, i.e. coils arranged at predetermined locations alongthe extracorporeal blood circuit.

[0013] The method described above has various drawbacks. In particular,although valid from the theoretical standpoint, this method is not ableto provide satisfactory results from the practical standpoint, becausethe high electrical impedance caused by the peristaltic pump, which infact interrupts the continuity of blood flow, necessitates operatingwith relatively high currents in order to make use of the scantconductivity of the materials of which the extracorporeal circuit, thedialyzer, the hose of the peristaltic pump and the bubble trap are made(PVC, polycarbonate). The use of relatively high currents is certainlynot advisable in a machine connected to a patient and even if they wereused, it would not be possible to transmit these high currents by meansof an inductive coupler which, among other things, also generatesparasitic currents which disturb the measurement. In some dialysismachines the bubble trap also represents a high impedance of the sameorder of magnitude as the peristaltic pump, and thus makes one of thedrawbacks previously described even more acute.

[0014] Therefore, in view of the fact that it is advisable to operatewith relatively low currents and that the impedance of the peristalticpump, and in the majority of cases, of the bubble trap, is high, itfollows that disconnection of one of the needles causes only slightchanges in current, such as could be confused with the background noiseof the measuring instrument.

[0015] Furthermore, this method does not take into account that thepatient might be connected to earth and that the dialyzer itself is infact connected to earth, since the dialysis fluid circuit is connectedto earth in accordance with the provisions of the safety standardsrelating to dialysis machines.

[0016] The aim of the present invention is to provide a method thatobviates the drawbacks of the prior art.

[0017] According to the present invention, a method is provided formonitoring the access to the cardiovascular system of a patientundergoing an extracorporeal treatment of blood in a machine comprisinga treatment device and an extracorporeal circuit having an arterialbranch and a venous branch, the arterial branch having a first andfitted with an arterial needle to be inserted in the vascular system ofthe patient and a second end connected to an inlet of the treatmentdevice, and the venous branch having a first end connected to an outletof the treatment device and a second end fitted with an venous needle tobe inserted in the vascular system of the patient, the method beingcharacterized in that it comprises the steps of:

[0018] generating a potential difference between a first point of thevenous branch and a part of the machine;

[0019] detecting the value (dV) of a quantity that correlates with theelectric current along at least one section of the venous branch betweenthe first point (B) and the venous needle; and

[0020] comparing the detected value (dV) with a reference range (I).

[0021] The present invention relates, in addition, to a monitoringdevice.

[0022] According to the present invention, a device is provided formonitoring the access to the cardiovascular system of a patientundergoing an extracorporeal treatment of blood in a machine comprisinga treatment device and an extracorporeal circuit having an arterialbranch and a venous branch, the arterial branch having a first endfitted with an arterial needle to be inserted in the vascular system ofthe patient and a second end connected to an inlet of the treatmentdevice, and the venous branch having a first end connected to an outletof the treatment device and a second end fitted with an venous needle tobe inserted in the vascular system of the patient, the device beingcharacterized in that it comprises:

[0023] a voltage generator for generating a potential difference betweena first point (B) of the venous branch and a part of the machine;

[0024] a detector for detecting the value (dV) of a quantity thatcorrelates with the electric current along at least one section of thevenous branch between the first point (B) and the venous needle;

[0025] calculating means for comparing the detected value (dV) with areference range (I).

[0026] The invention will now be described, with respect to the appendeddrawings, in which:

[0027]FIG. 1 is a schematic representation of a dialysis machineconnected to a patient and equipped with a monitoring device accordingto the invention;

[0028]FIG. 2 is a schematic representation of a dialysis machineconnected to a patient and equipped with a variant of the device in FIG.1; and

[0029]FIG. 3 is a schematic representation of a dialysis machineconnected to a patient and equipped with another variant of the devicein FIG. 1.

[0030] In FIGS. 1, 2 and 3, the number 1 indicates a dialysis machineconnected to a patient P. The machine 1 comprises an extracorporealblood circuit 2 and a dialysis fluid circuit 3 which pass through adialyzer 4, which comprises a blood compartment 5 and a dialysiscompartment 6 separated by a semipermeable membrane 7.

[0031] The extracorporeal blood circuit 2 comprises, in addition to thecompartment 5 of dialyzer 4, an arterial branch 8, along which aperistaltic pump 9 is arranged, supplying a blood flow Q_(b), and avenous branch 10, to which a bubble trap 11 is connected. Arterialbranch 8 has a needle 12 which, in use, is inserted in a fistula ofpatient P to collect blood from the cardiovascular system of the patientP, while venous branch 10 has a venous needle 13 which, in use, isinserted in the fistula for returning the treated blood to thecardiovascular system of patient P. Arterial and venous branches 8 and10 are tubes made of a plastic material, generally PVC, as well asbubble trap 11. Dialyzer 4 is also made of plastic material, the housingof it generally of polycarbonate.

[0032] In FIGS. 1, 2 and 3, machine 1 is equipped with a device 14 fordetecting disconnection of needles 12 and 13. The principle of thedevice 14 is based on the electrical conductivity of the blood and ondata found experimentally that showed that circuit 2 made of PVC can beregarded as an insulator and that both the peristaltic pump 9 and thebubble trap 11 can be regarded as concentrated impedances designated Z1and Z2 respectively. Peristaltic pump 9 cyclically interrupts the bloodflow Q_(b), at each half-turn of the pump 9 and accordingly theimpedance Z1 is a function of the number of turns of pump 9 and of thesupply frequency. Experiments have established that the impedance Z1 isbetween 500 and 2000 kΩ. The impedance Z2 is determined as well by thefact that the blood flow Q_(b) is interrupted in the bubble trap 11 andthe value assumed by Z2 is also between 500 and 2000 kΩ. Recent designsof bubble trap have a relatively low impedance, which is negligible withrespect to the impedance Z1. This circumstance will be borne in mindwhen describing the operation of device 14.

[0033] The impedances of the other compartments of extracorporealcircuit 2 are negligible with respect to the values of impedance Z1. Toevaluate the operation of device 14, it is necessary to bear in mindthat the dialyzer 4 is connected to earth via the dialysis fluid circuit3 and that the patient P may be connected to earth (R=0) or insulated(R=infinity) or in a situation intermediate between the two precedinglimiting situations. These distinctions are important as it would bedifficult to prevent the patient P from moving, for example resting afoot on the floor or placing a hand on the bedhead of an uninsulatedbed, thus altering the configuration of the possible electric circuitsdefined by the machine 1, the patient P, and device 14.

[0034] In FIG. 1, device 14 comprises a control unit 15, a generator 16connected to arterial and venous branches 8 and 10, a detector 17 of avoltage drop and a clamp 18 arranged along venous branch 10. Generator16 is connected respectively by two conductors 19 and 20 and by twocapacitive couplers 21 and 22 respectively to arterial branch 8 and tovenous branch 10. Detector 17 is connected by two conductors 23 and 24and by two capacitive couplers 25 and 26 to venous branch 10 fordetecting the voltage drop along a predetermined section 10 a of branch10. An optimum filter FO is arranged along conductors 23 and 24 forminimizing the effect of noise on the input of detector 17, which isconnected to control unit 15 for transmitting a value dV indicating thevoltage drop in section 10 a to unit 15, which compares this value witha threshold value dVT. If the value detected is not inside a range Iaround the threshold value dVT, control unit 15 emits a control signal Sfor stopping pump 9, a control signal G for closing clamp 18 and asignal T for emitting a visible and/or acoustic alarm signal.

[0035] Capacitive couplers 21, 22, 25 and 26 are made with respectivemetal tubes 27, which are connected to the respective conductors 19, 20,23 and 24 and are arranged around portions of the respective PVC tubes.From the electrical standpoint, tube 27 defines a first plate of acapacitor, the PVC tube defines the dielectric, and the blood inside thePVC tube defines the second plate.

[0036] Capacitive coupler 21 is arranged on arterial branch 8 at a pointA between the arterial needle 12 and the peristaltic pump 9, whilecapacitive coupling 22 is arranged on the arterial branch 10 at point Bbetween bubble trap 11 and needle 13. Detector 17 is connected to venousbranch 10 at points C and D, both of which are between point B and thevenous needle 13 and define the end of section 10 a.

[0037] When the patient is insulated (R infinite) and impedance Z2 ishigh, the operation of the monitoring device 14 according to theinvention is as follows: the blood being circulated in theextracorporeal circuit 2 in the direction indicated by the arrows inFIG. 1, a potential difference is applied between venous branch 10 andarterial branch 8 by means of generator 16 and the respective capacitivecouplers 21 and 22. This potential difference generates a currentbetween a section of venous branch 10 and a section of arterial branch 8which is closed by the cardiovascular system of the patient P on accountof the high impedances Z1 and Z2 on the other portion of circuit 2.Detector 17 detects the voltage drop along section 10 a of arterialbranch 10 and stores a value indicating the voltage drop as thresholdvalue dVT, determines the range I of acceptability around the thresholdvalue dVT and checks whether the successive values dV are inside rangeI.

[0038] When one of the needles 12 and 13 accidentally becomesdisconnected from the fistula, the detector 17 detects the cancellationof the voltage drop dV in section 10 a, and the control unit 15 emitssignals S, G and T for stopping the peristaltic pump 9, closing theclamp 18, and emitting an alarm signal.

[0039] The monitoring device 14 is particularly advantageous because itby-passes the impedance Z1 and Z2, and the dialyzer 4 which is connectedto earth. Therefore it is possible to work with relatively low currentssince disconnection of one of the needles 12 and 13 represents anappreciable change in the current along a circuit comprising a portionof the arterial branch 8 and a portion of the venous branch 10, theconductors 19 and 20 and the cardiovascular system of the patient P.

[0040] When the patient P is connected to earth (R=0), if the venousneedle 13 becomes disconnected, there is no current flowing through thevenous branch 10 and therefore detector 17 detects a voltage drop equalto zero as is the case when patient P is insulated. If the arterialneedle 12 becomes disconnected, there is a voltage drop in section 10 a,which is a function of the impedance Z1 of the peristaltic pump and istherefore significant owing to the high value of impedance Z1.

[0041] When Z2 is negligible, disconnection of venous needle 13 isdetected both when the patient is insulated (R infinite) and when he isconnected to earth (R=0), as preferential flow of current occurs alongthe portion of extracorporeal circuit 2 on the side of patient P.

[0042] In the embodiment of FIG. 2, there is no capacitive coupler 25since point C coincides with point B, whereas point D is located closeto the venous needle 13. In this case, detector 17 detects the change involtage along a section 10 b, which is a section of branch 10 betweenpoint B (i.e. C) and, essentially, the venous needle 13.

[0043] When the patient P is insulated (R infinite) and Z2 is high, thecurrent circulates through conductors 19 and 20, a portion of the venousbranch 10 and a portion of the arterial branch 8. Disconnection of oneof the needles 12 and 13 has the effect that the voltage drop iscancelled along the section 10 b and the patient P.

[0044] When the patient P is connected to earth (R=0), a disconnectionof the venous needle 13 causes the cancellation of the voltage drop asin the proceding case, whereas when the arterial needle 12 isdisconnected, the voltage drop becomes a function of the impedance Z1 asin the preceding case.

[0045] When Z2 is negligible, the considerations relating to the variantin FIG. 1 apply, except that the greater length of section 10 b relativeto section 10 a makes it possible to refer to high values dV, at equalcurrent passing through the venous branch 10, and therefore the device14 is more efficient, as it increases the difference between the valueof the voltage drop dV determined by the condition with the venousneedle 13 connected and the zero value of dV.

[0046] According to a variant that is not shown, again the capacitivecoupler 26 is omitted and is replaced with a conductive bracelet, notshown, connected directly to one wrist of the patient P. The operationof the said variant that is not shown does not differ substantially fromthe variant in FIG. 2.

[0047] According to the variant in FIG. 3, capacitive coupling 21 to thearterial branch 8 is omitted, since generator 16 is connected to earthvia conductor 19, and detector 17 is connected to the venous branch 10via conductor 23 and the capacitive coupler 25 at point C and to thearterial branch 8 via conductor 24 and the capacitive coupler 26 at apoint F between the peristaltic pump 9 and arterial needle 12.

[0048] In use, when the patient P is insulated (R infinite) and theimpedance Z2 is high, the value dV of voltage drop along section 10 c ofthe venous branch, section 8 c of the arterial branch 8, and thecardiovascular system of the patient P is detected. Section 10 c isbetween point C and arterial needle 13, whereas section 8 c is betweenpoint F and venous needle 12. Disconnection of one of the needles 12 and13 causes cancellation of the voltage drop.

[0049] When the patient P is connected to earth (R=0), a disconnectionof the venous needle 13 causes the cancellation of the voltage drop,whereas a disconnection of the arterial needle 12 does not cause anyappreciable change in the voltage drop dV.

[0050] When the impedance Z2 is negligible, a low current will passalong section 10 c, however section 10 c along which the voltage drop dVis determined is relatively long and therefore a detection thereof issignificant.

[0051] In practice, all the variants of the monitoring device 14described with reference to the FIGS. 1, 2 and 3 enable a reliabledetection of the disconnection of the venous needle 13, since adisconnection of the venous needle 13 causes, both when the patient P isinsulated (R infinite), and when the patient is connected to earth(R=0), a significant change in the value dV of voltage drop, incomparison with the situation in which the venous needle 13 isconnected.

1. Method for monitoring the access to the cardiovascular system of apatient undergoing an extracorporeal treatment of blood in a machine (1)comprising a treatment device (4) and an extracorporeal circuit (2)having an arterial branch (8) and a venous branch (10), the arterialbranch (8) having a filter end fitted with an arterial needle (12) to beinserted in the vascular system of the patient and a second endconnected to an inlet of the treatment device (4), and the venous branch(10) having a first end connected to an outlet of the treatment device(4) and a second end fitted with an venous needle (13) to be inserted inthe vascular system of the patient, the method being characterized inthat it comprises the steps of: generating a potential differencebetween a first point (B) of the venous branch (10) and a part of themachine (1); detecting the value (dV) of a quantity that correlates withthe electric current along at least one section (10 a; 10 b; 10 c) ofthe venous branch (10) between the first point (B) and the venous needle(13); and comparing the detected value (dV) with a reference range (I).2. Method according to claim 1, characterized in that the first point(B) is located between the venous needle (13) and a bubble trap (11)connected to the venous branch (10).
 3. Method according to claim 1,characterized in that the quantity that correlates with the current is avoltage drop.
 4. Method according to claim 1, characterized in that itcomprises the step of emitting at least one control signal (S, T, G)when the detected value (dV) is outside the reference range (I). 5.Method according to claims 2 and 3, characterized in that the voltage isgenerated by means of a generator (16) and detection is by means of avoltage drop detector (17), the generator (16) and the detector (17)being connected to the extracorporeal circuit (2) by means of capacitivecouplers (21, 22, 25, 26).
 6. Method according to claim 5, characterizedin that a potential difference is generated between the first point (B)and the arterial branch (8) at a second point (A) located between thearterial needle (12) and a peristaltic pump (9) arranged on the arterialbranch (8).
 7. Method according to claim 6, characterized in that thevoltage detector (17) is connected to the venous branch (10) at a thirdand fourth point (C, D) located between the first point (B) and thevenous needle (13), the section (10 a) of the venous branch (10) wherethe detection takes place being located between the third and fourthpoint (C, D).
 8. Method according to claim 6, characterized in that thevoltage detector (17) is connected to the venous branch (10) at a thirdpoint (C) coinciding with the first point (B) and at a fourth point (D)close to the venous needle (13), the section (10 b) of the venous branch(10) where the detection takes place being located between the thirdpoint (C) and the venous needle (13).
 9. Method according to claim 5,characterized in that a potential difference is generated between thefirst point (B) and a part of the machine (1) connected to earth. 10.Method according to claim 9, characterized in that the voltage dropdetector (17) is connected to the third point (C) and to the arterialbranch (8) at a fifth point (F) located between the peristaltic pump (9)and the arterial needle (12), the section (10 c) of the venous branch(10) where the detection takes place being located between the thirdpoint (C) and the venous needle (13).
 11. Device for monitoring theaccess to the cardiovascular system of a patient undergoing anextracorporeal treatment of blood in a machine (1) comprising atreatment device (4) and an extracorporeal circuit (2) having anarterial branch (8) and a venous branch (10), the arterial branch (8)having a first end fitted with an arterial needle (12) to be inserted inthe vascular system of the patient and a second end connected to aninlet of the treatment device (4), and the venous branch (10) having afirst end connected to an outlet of the treatment device (4) and asecond end fitted with an venous needle (13) to be inserted in thevascular system of the patient, the device (14) being characterized inthat it comprises: a voltage generator (16) for generating a potentialdifference between a first point (B) of the venous branch (8) and a partof the machine (1); a detector (17) for detecting the value (dV) of aquantity that correlates with the electric current along at least onesection (10 a; 10 b; 10 c) of the venous branch (10) between the firstpoint (B) and the venous needle (13); calculating means (15) forcomparing the detected value (dV) with a reference range (I).
 12. Deviceaccording to claim 11, characterized in that the first point (B) isbetween the venous needle (13) and a bubble trap (11) arranged along thevenous branch (10).
 13. Device according to claim 11, characterized inthat the detector (17) is a voltage drop detector (17).
 14. Deviceaccording to one of the claims from 11 to 13, characterized in that thecalculating means (14) is designed to emit at least one control signal(S, T, G) when the detected value (dV) is outside the reference range(I).
 15. Device according to claims 12 and 13, characterized in that thegenerator (16) and the detector (17) are connected to the extracorporealcircuit (2) by means of capacitive couplers (21, 22, 25, 26).
 16. Deviceaccording to claim 15, characterized in that each of the capacitivecouplers (21, 22, 25, 26) comprises at least one metal tube (27) woundaround a respective portion of the extracorporeal circuit (2). 17.Device according to claim 15, characterized in that the generator (16)is connected to the venous branch (10) at the first point (B) and to thearterial branch (8) at a second point (A) located between the arterialneedle (12) and a peristaltic pump (9) arranged on the arterial branch(8).
 18. Device according to claim 17, characterized in that thedetector (17) is connected to the venous branch (10) at a third andfourth point (C, D) located between the first point (B) and the venousneedle (13), the section (10 a) of the venous branch (10) where thedetection takes place being located between the third and fourth point(C, D).
 19. Device according to claim 17, characterized in that thedetector (17) is connected to the venous branch (10) at a third point(C) which coincides with the first point (B), and at a fourth point (D)which is located close to the venous needle (13), the section (10 b) ofthe venous branch (10) where the detection takes place being locatedbetween the first point (B) and the venous needle (13).
 20. Deviceaccording to claim 15, characterized in that the generator (16) isconnected to the venous branch (10) at the first point (B) and to a partof the machine (1) connected to earth.
 21. Device according to claim 20,characterized in that the detector (17) is connected to the venousbranch (10) at a third point (C) and to the arterial branch (8) at afourth point (F), the fourth point (F) being located between theperistaltic pump (9) and the arterial needle (12), the section (10 c) ofthe venous branch (10) where the detection takes place being locatedbetween the third point (C) and the venous needle (13).